Layers formed on or over a substrate are a common element of thin-film devices and backplanes. Such layers are found, for example, in displays, touch screen, and optical films. In some applications, high-aspect-ratio structures having a depth or thickness much greater than a width are useful. For example, micro-louver structures desirably having a reduced viewing angle but a high transparency use high-aspect-ratio light-absorbing structures to form a privacy screen. Optical structures such as phase and amplitude gratings, plasmonic devices, and wire-grid polarizers also use structures with a small width.
Most thin-film devices are constructed using photolithographic techniques in which light-sensitive layers are coated on a surface, exposed through a mask to cure a patterned portion of the light-sensitive layer, and then chemically treated to remove the cured or uncured portions of the light-sensitive layer and layers coated beneath the patterned light-sensitive layer. Very small features, for example, on the order of 500 nanometers to many microns are made using such techniques. Even smaller structures are made using higher-frequency radiation (e.g. ultra-violet radiation) as is well-known in the semiconductor industry. These semiconductor processes tend to be expensive, slow, and limited in size. Furthermore, using such techniques it is often difficult to form high-aspect-ratio structures with a rectangular cross section rather than a structure with undercut edges having a roughly trapezoidal shape.
In one prior-art method described in U.S. Patent Publication 2008/0144179, privacy screens are made by coating a layer of photo-sensitive resin on a first substrate. A mask is used to pattern the photo-sensitive resin. The mask has a pattern corresponding to the arrangement of light-absorbing and transparent material. The pattern is etched into the exposed resin and a layer of curable material is coated over the photo-lithographically etched resin in a vacuum. The curable material is etched to expose the photo-sensitive resin layer, and cured. A transparent second substrate is then laminated to the resin layer. Alternatively, the second substrate is laminated after the resin is etched and curable material wicked into the etched areas using capillary forces, and cured. In yet another method, multiple resin layers having etched areas are laminated together forming gaps and curable material wicked into the gaps using capillary forces, and cured. These methods are limited in the depth they can achieve since photo-lithographic etching has a practical depth limitation or the patterns available are limited to those that can support etching. Furthermore, photo-lithographic processes are relatively expensive and slow.
In other prior-art methods described in U.S. Pat. No. 3,524,789 entitled “Louvered transparent sheeting made by skiving”, alternating layers of light-absorbing material and light-transparent material are laminated together. Such layers are formed by extrusion or by laminating pre-formed sheets together, as is also described in European Patent Application Number 91306224.6 entitled “Method of making a flexible louvered plastic film”. The laminate is then cut into cross-sectional portions, each portion forming a micro-louver sheet with micro-louvers. Alternatively, alternating layers of light-absorbing material and light-transparent material are formed in cylinders and laminated together. Thin micro-louver sheets are cut from the cylinder with a knife.
These approaches use relatively thick layers of light-absorbing material and light-transparent material that limit the transparency of the resulting micro-louver sheet. It is also difficult to make large micro-louver sheets since it is difficult to cut large, thin sheets, for example using skiving. Furthermore, such sheets typically need additional processing to remove curl and polish the edges.
Imprinting methods generally known in the prior art are an alternative way to make structures in a layer. Such methods typically include coating a curable liquid, such as a polymer, onto a rigid substrate. The polymer is partially cured (through heat or exposure to light or ultraviolet radiation) and then a pattern of micro-channels is imprinted (embossed or impressed) onto the partially cured polymer layer by a master having a reverse pattern of ridges formed on its surface. The polymer is then completely cured.
U.S. Patent Application Publication No. 2009/0242110 describes a method for manufacturing a polarizer that includes transferring a ridge-trough pattern with a mold onto a surface of a substrate formed with a transparent medium, forming a metal layer so as to at least fill a trough portion of the ridge-trough pattern transferred on the substrate, grinding the metal layer and a ridge portion of the ridge-trough pattern transferred on the substrate to form a periodic pattern of a material of the metal layer and the transparent medium, where a period of the ridge-trough pattern is not longer than a wavelength of an incident light flux. Alternatively, U.S. Patent Application Publication No. 2011/0303404 discloses a conformal, multilayer micro-channel structure having a wear-resistant interior micro-channel surface coating of an ALD deposited conformal alumina (Al2O3) ceramic of about 1000 Angstroms in thickness and a titanium nitride (TiN) of about 300 Angstroms to about 1000 Angstroms in thickness. The Al2O3/TiN multilayer structure is resistant to erosion and to electro-chemical corrosion as is found in prior-art micro-channel coolers and structures.
Attributes such as transparency, contrast, or reflectivity are important for optical systems. Overall thickness and cost are also important device attributes.